Magnetic induction devices (e.g., electrical transformers, chokes, and suchlike) are designed to transfer electrical energy between inductively coupled wound conductors (coils) based on the mutual induction effect. For example, in electrical transformers the alternating electrical current supplied to a primary winding inductively coupled to the transformer' core creates a magnetic flux in the core which induces electric motive force (EMF) or voltage in a secondary winding inductively coupled to the transformer' core.
A three-phase transformer typically comprises a magnetic-core circuit and three coil blocks inductively coupled to the magnetic-core circuit. Each one of the coil blocks usually consists of primary and secondary windings. State of the art three-phase electrical transformers usually utilize the so called “E+1” magnetic core configuration (where coils are mounted over the three legs of the “E”-shaped frame of the magnetic core that is thereafter closed by the “1”-shaped yoke of the core). The “E+1” magnetic core configuration provides a planar core structure, comprised of several interconnected magnetic core yoke and leg elements geometrically arranged in a single plane.
For example, U.S. Pat. No. 6,668,444 discloses a three-phase transformer having a flat magnetic core configuration made from an amorphous metal strip. This flat magnetic core configuration utilizes “stair-stepped” joints designed to facilitate the opening of the core legs for lacing coils over them, and thereafter closing the joints to thereby close the magnetic core circuit. This manufacturing technique however provides a flat magnetic-core structure which is less efficient for magnetic flux distribution, requires complex technologies of magnetic circuit closure, and results in substantially high weight magnetic cores. In particular, it is impossible to resolve the problem of asymmetrical magnetic flux distribution in such flat magnetic-core structures of these flat transformer configurations.
Possible alternatives to the flat three-phase transformer configurations are the triangular type magnetic-core magnetic systems. For example, U.S. Pat. No. 6,683,524 discloses a three-phase transformer having a triangular (delta) structure. In this solution the transformer core is made of three frames, each comprising several rings wound from a strip of magnetic material of constant width. The frames are assembled into a core such that two triangular yoke structures are formed having vertical legs extending between of their corners, wherein the legs are formed from the wound rings which are slid over, offset or splayed one relative to the other. This configuration provides transformer legs having a polygonal cross-section shape, but it is however very complex to manufacture, and its structural configuration increases the magnetic losses.
US Patent publication No. 2010/0194515 describes a triangular three-phase transformer constructed from three frames which are assembled to construct hexagonal legs (also known as ‘hexaformer’) employing tapered rings structures obtained using an off-set wound technique. It is suggested in this publication to fabricate the core frames partly from wound amorphous ribbon and partly from electrical steel, which is extremely difficult because these materials have different thicknesses, different mechanical strengths and require different effort tensions during winding. Therefore, such construction of the frames does not provide a high winding density, which is one of the main parameters of the magnetic system. Furthermore, the use of such hybrid core frames increases load losses due to increased magnetic losses in electrical steel compared to amorphous materials. This publication further suggests mechanically stretching the core frames, which is very problematic since the required efforts are determined by the volume of the electrical steel used in the frames. Furthermore, the simultaneous displacement of the amorphous ribbon and electrical steel by these efforts is prone to breakage of the amorphous metal ribbon, which in turn will lead to an increase in no-load current.
European patent publication No. EP 2,395,521 discloses a method for manufacture of triangular transformer cores made of amorphous metal ribbon, wherein the legs of the magnetic core are arranged in a triangular configuration where the cross-section of the core legs has a circular or polygonal shape. In order to obtain the required cross-sectional shape of the legs the core frames are constructed from layers of continuously wound band, where the width of the bands is adjusted according to the respective layer of the core leg by means of laser cutting. However, molten material that is typically formed during such laser cutting of amorphous ribbon, results in stark molten drops of ribbon material formed along the cutting edges which causes gaps between the layers of the magnetic ribbons during their winding. In addition, such stark molten drops may also create conditions for the occurrence of short circuits in the operation of the magnetic system. It is noted that such a method of manufacturing the magnetic core with variable cross sections is very complex and problematic to realize.
U.S. Pat. No. 6,809,620 discloses three-phase transformers having a triangular cage core structure assembled from three frames. The three frames assembly form triangular yoke structures whose corners are connected by three legs, where the core frames are wound from a plurality of strips, each of the strips being offset from adjacent strips to obtain rhomboidal cross-section of the frames. The magnetic core is made from interleaved ring structures made of wires or strips of magnetic material, wherein each of the rings makes up part of two of the legs. However, the interleaved rings structure suggested in this patent necessitate very complex production technology, in particular for manufacture of power transformers.